Electric discharge device



June 14,' 1960 s. E. wEBBx-:R

ELECTRIC DISCHARGE DEVICE Filed July 25, 1955 United States Patent C) ELECTRIC DISCHARGE DEVICE Stanley E. Webber, Schenectady, NX., assgnor to General Electric Company, acorporaton of New York Filed July 25, 1955, Ser. No. 524,002

2 Claims. (Cl. S15- 3.5)

My invention relates to improved electric discharge devices of the traveling wave type.

In the class of electric discharge devices generally known as traveling wave tubes, an electron beam is passed in energy-exchanging relation with the high frequency field of a slow wave structure, i.e., a conductive structure along which an electromagnetic wave is propagated at a phase velocity, in the direction of the beam travel, which is slow compared with the velocity of light. The phase velocity is actually very close to the velocity of the electron beam or the group velocity of the hunched electrons of the beamso that energy may be exchanged in one direction or vthe other between the beam and the high frequency eld throughout all or a substantial part of the length of the slow wave structure.

In the operation of devices of the above type as amplitiers, it is known that self oscillations tend to occur and that, unless steps are taken to minimize these oscillations, the device becomes useless as an amplifier. It has accordingly become common practice to employ an attenuator located between the input and output ends of the slow wave structure for the purpose of reducing the amplitude of the waves propagated along the structure. Hence, any waves which pass through the region of the attenuator more than once are reduced to a greater degree and the undesirable tendency of the device toward self oscillation is minimized or eliminated. Attenuators frequently used include a coating or body of relatively lossy material with respect to high frequency energy supported in prox.

imity to the slow wave structure intermediate its input and output ends. These coatings have been applied t longitudinal supporting rods which are frequently used to support the slow wave structure or, in cases where a glass envelope is employed, a coating may conveniently be applied to either the inner or outer wall of the envelopel intermediate the ends of the slow wave structure.

Ihave found that the presence of an attenuator changes the phase velocity of the wave along the slow wave structure. If the velocity of the electron beam and the geometry of the slow wave structure are such as to provide optimum interchange of energy between the beam and the structure in the absence of the attenuator, this change in phase velocity tends to reduce the gain and eiiiciency of the tube. In accordance with `an important aspect of my invention, I provide structures which compensate for the elect of the attenuator on the wave velocity and in this way maintain the optimum energy-exchanging relation `between the electron beam and the traveling wave of the slow wave structure.

In one specic embodiment in which the attenuator tends to increase the phase velocity along the axis of the helix, the pitch of the helix is increased in the region of the attenuator to reduce the resultant phase velocity to that which would have existed in the absence of the attenuator. In order to minimize reections, the attenuator is preferably tapered at its ends and accordingly the change in pitch at the ends of the `attenuator is gradual. In a modied form of my invention, this same etect is producedV by slowing the velocity of the Wave along the con- The collector electrode, outer conductor 13 and also the ductive path of the slow wave structure. This may be done, for example, by providing circumferential grooves .in the helix, a very common form of slow wave structure, which tend to increase its inductance and consequently reduce the phase velocity of the wave. In another embodiment of the invention, the character and location of the attenuator is such that it tends to decrease the phase velocity and in this embodiment of my invention the compensation is effected by decreasing the pitch of lthe helixv in the central portion ofthe attenuator. Y

In accordance with the foregoing, it is an important object of my invention to provide an improved traveling' wave tube including an attenuator which is characterized by improved gain and efficiency.

It is another object of my invention to provide means for compensating for the eiect of an attenuator on phase velocity in a traveling wave tube.

Further objects and advantages of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. l is au elevational view in section of a traveling wave tube embodying my invention.

Fig. 2 is an elevational view in section of a portion of a device similar to that illustrated in Fig. 1 and embodying a modication of my invention,

Fig. 3 is an enlarged elevational view of a Wire suitable for use in the helix of Fig. 2, and

Fig. 4 is a sectional view taken along the line 4-4 of Fig. 3.

Referring now to Fig. l of the drawing, I have shown my invention embodied in a traveling wave tube including formed of glass.

an electron beam along the axis of the envelope and at the other end is supported a suitable collecting electrode 3 including a lead-in conductor 4. `The electron gun iucludes a source of electrons in the form of a generally cylindrical cathode 5 which may be heated to electron',

emitting temperature by a resistance heater illustrated at 6. Electrons from the cathode 5 are accelerated and focused by a suitable, vgenerally cylindrical anode 7. While my invention may be applied to any type of slow wave structure, I have illustrated a simple helix 8 as extending axially of the envelope and terminating at its opposite ends respectively in spaced relation to the accelerated electrode 7 and the collecting electrode 3. The ends of the helix are brought out through the envelope by means of conductors 9 and 10 which form the inner conductors of input and output concentric transmission lines respectively whichv are completed by outer conductors 11 and 12. The conductors ll and I2. are joined to a cylindrical conductor 13 which surrounds the envelope l in spaced relation thereto and extends a little beyond the helix at each end of the tube. lt will be understood that, in operation of the device, a collimating axial magnetic eld is provided by means of suitable permanent magnetic4 or electromagnetic means. A suitable electromagnetic field producing means is shown schematically as a coil 14'. For this reason the cylinder 13 is formed of non-magnetic material, such as copper or stainless steel. While a simple concentric line type of input and output connection has been illustrated, it will be apparent to those skilled in the art that other forms of coupling with the slow wave structure may be provided, if desired.

to the cathode and the positive terminal is grounded.

Patented June 14, w50 A helix Sfareoperated atvdirect currentground potential,

The accelerating electrode 7 is preferably operated at Y an adjustable direct current' voltage provided by the tapf16on ,the resistor'l'Sz:

Asvfkwillr'be Well understood byrthose'familiar withA the-operation-of'traveling vvaveV tubes, a high frequency V'signal impressed 'ontheinput circuit including conductors V9,and=,11=will appear at Vthe Voutput circuit including cenductors A1i) yand'12 inf amplified form, provided the velocity-ofthe beam passing'vfrom the electron gunto,A

th,ecollector lel ectro,de is suitably acljustedrbyV means of the' 'ppsitioryot"Y tap '16 with`v respectY tothe geometry of the Vslow Ywave structure, Yspecilically4 the helix 3 inV the illustrated'embodimentf/ t-is alsogwell'known that these structures tend to oscillare vunless somefmeans is pro- 'vided for-, attenuating-any- Waves Ywhich tend to malte multiple excursions alongthe-V length ofthe helix@ As illustrated irl-Eig. lrcithe drawing, tbismay-'be accomplished by ,ajayer ofi lossy material in the formY of coating 17 applied to therexteriorY of the envelope: in the form' ofea cylindrical path.V i The coating may be aquadag, fori example, and` 'of' athicknessY which will give the desired surfacerresistance and hence a desired amount of attenuation. The resistancemay have a value of lllQQ ohms persquare unit of` surface area. The ends of the; coatingare preferably taperedairl thickness,

Vas,illustrated*to minimize reliections.- If thepitch of the'helix were to remainy constant throughcutwthe length of the helix and at a value which would be optimum, consideringthe velocity of the elect-ron beam as estab- V lishedv` bfy/the direct currentrrpotentialfgthey presence of the attenuator vwould tendto changethe. phaseV velocity illustrated more clearly in enlarged Figs. Sand ,4,vthe,

VVvvire from which this portion of the helix is wound is shaped to provide circumferential grooves 19 at spaced points along the length of the wire and thereby provide discrete areas of reduced cross-section which increase the inductance of the helix Wound'from this Wire. Y

Y While I have shown `and kdescribed particular embodiments of my" invention, 'it'willbe apparent to' those skilled t in the art ythat changes rand modifications may be made Without departingVV from the invention inY its broader' aspects and l airnrinv the appended claims to ,cover all such changes' andjmodications 'asfalt-within" the true Yspirit and scope oflmy invention.

What I claim as new vandwdesire to secure by Letters Patent of the United States is: l Y

, l. A high frequency` ampliler ofr the traveling r,wave

Y type comprising 'a slov/"wave'transniission circuit havingV an'input end VandV an .output end,rnejans impressing'a WaveV to Vbe amplified upon the'f input end .of said circuit to initiateV travel ofthe Wavestherealong, electrode'mean's" projecting anelectron stream at 1a predetermined average velocityY along at least aY portion of said circuit inthe' i direction of Wave travel. and in4 the electric field assol ciated with said traveling waves, said slow Wavecircuit' having a waveV propagating characteristic' making the velocity of said waves therealong inthe regionjofisaicl1Y electron' stream substantially the same as said predetermined average'velocity'of saidstream, high frequencyl'o'ss along the helix and cause operation of the rdevice at less;V

' than, theymaximum gain and efficiency.V I have found that-fV by Ypreperly compensating lforA this change' in Vphase velociiy, the' adversejeect of .the attenuater on gain Vmo'dlcation shown in Fig.y l, theV attenuator increases thev phase 'velocity' as' compared With'the phasevelocity withoutA anattenuator overfa substantial range oftatt'e'numaterial supported inthe region of said lield intermediate the ends of said slow waveA structure forV reducinggthe tendency of said circuit toward self oscillation andtend-- ing to increase the WaveV velocity along said slovvVV wave.. circuit, said 'slow Wave circuit having discrete a'rea's offV reducelrcrossk section Vat spaced pointsY alon'grits length in the region of said loss material to modify the propagarV tion characteristic of said circuit,y in the regicn'of saidf" loss, material and compensatefor thefefecton'thevelccityA i Y ofv'qa've transmission of said loss'ma'terial.

ato' re'sis'tancejvalues. Accordingly, Ythe pitch of the Y helix is incre'sedin the registrar theiraenuatefffo ZIA high frequency amplifier offV the traveling'vvave Vtype comprising a helix having an input end'rand an outf" put end, meansimpressing a Wave to be amplified uponVV l .the input end of said helix to initiate. travell of the waves maintain thels'arhe" relationship' betyveen the traveling Y Y wave and the electron bear'nas wouldexist'initheabsence oftlie' attenu'ator. Since thethickn'essofthe'attenuator t increases gradallythe lincrease''in pitchof theVV helix has"been{'made"gradual,` It Will'be appreciated that`th'e length ofthe attenuator andthe amount'Y of increase'in t thpitch of the helix havelbfe'e'n exaggerated to emphasize 'applicants invention; Y

The Yelect of the" atte'nurtoi"Y on the y phasev'elocity I of theyhgli frequency wave' 'ofl the slow wave structure varies 'withj the surface' resistance of the attenuator, Vits,

physical location'and the operating frequencies of the'j device; The' present invention contemplates al compensation for, the effect of the attenuatoron phase or WaveV velocity after the other' operating conditions have been established; In the modification shown in Fig. 1, the at'tenuat'or4 resistance may be considered' as 'separated fromjthe h elix by distributed'shunt capacitance provided bythe glass envelope. lf the attenuatorv is located within thev envelope'Y and in proximity to the helix, it tends'to reducethe phase velocity of' the high frequency Wave on the'y slovv' wave structure; It is apparent that this effecty on phase velocitymay be compensated for by compressing turns'of the helix, i.e.', reducing the pitch in the regionv Y oftheattenuator. VVThe same end result mayalso be Yobtained*by reducingth'e wavelvelccity along therhe'lixY l byj'inercasing the inductarneev thereof; lnY Figs'. 2 4, in-

clusive@ 'have shown a modification 'of my inventionV havin'g'fanv attenuatcrV consisting"y of4` a hollow4 cylindrical bddyfI-T'gof ,lossy insulating' material surrounding the 'helf V8' and (located inside thewall of theenvelopeY i; Th -deme'ase inphase velocity thatY would result from atte'nufatr is co'rnpuensaterl by increasingthe'induct.`

ance of the helix in the region of the attenuator. As

'therealong`, electrode means projecting anV electron streja'nrV at apredetermined average velocity alongza't least a p'cil tiofn of said helix the jdirectionl Lof wave travel the electric field associated with 1 said traveling Waves",l said helix having a Wave propagating characteristic'mak-v 1 Ying' the velocity of said waves therealong' in- 'therreginof` References Cited in the le of this 'patent UNITED STATES PATENTS 

